Thermoelectric Properties of Ca-Filled CoSb3-Based Skutterudites Synthesized by Mechanical Alloying

Ca (z) Co4-x (Fe/Mn) (x) Sb-12 skutterudites were prepared by mechanical alloying and hot pressing. The phases of mechanically alloyed powders were identified as gamma-CoSb2 and Sb, but they were transformed to delta-CoSb3 by annealing at 873 K for 100 h. All specimens had a positive Hall coefficient and Seebeck coefficient, indicating p-type conduction by holes as majority carriers. For the binary CoSb3, the electrical conductivity behaved like a nondegenerate semiconductor, but Ca-filled and Fe/Mn-doped CoSb3 showed a temperature dependence of a degenerate semiconductor. While the Seebeck coefficient of intrinsic CoSb3 increased with temperature and reached a maximum at 623 K, the Seebeck coefficient increased with increasing temperature for the Ca-filled and Fe/Mn-doped specimens. Relatively low thermal conductivity was obtained because fine particles prepared by mechanical alloying lead to phonon scattering. The thermal conductivity was reduced by Ca filling and Fe/Mn doping. The electronic thermal conductivity was increased by Fe/Mn doping, but the lattice thermal conductivity was decreased by Ca filling. Reasonable thermoelectric figure-of-merit values were obtained for Ca-filled Co-rich p-type skutterudites.